Restores the one-shot settleRun coverage in its own file beside the helper,
covers fork's seed contribution, the post-transfer rollback, the descriptor
model route on cold resume, manager-unload drain, and a failing teardown branch.
Removes three redundant checks the surrounding contracts already own: the
duplicate-Activation and live-id pre-checks (AgentRegistry.enter is the
authoritative collision boundary) and a rollback lifecycle edge that could never
publish because the epoch had no start edge.
Dudu-0223
bc504195df